This invention pertains to refractories, and particularly to refractories containing magnesium aluminate.
Magnesium aluminate and magnesium aluminate bonded refractory shapes are well known. Among their advantages, particularly compared with other basic refractories such as all magnesia brick, are better thermal shock resistance, lower thermal expansion, and lower thermal conductivity. Also a magnesium aluminate refractory shape has an advantage over a chrome-containing refractory in the growing number of applications in which chromium is considered a contaminant.
Despite their known advantages, magnesium aluminate refractories are not widely used, principally because of the difficulty of forming a dense, strong shape from them. The main reason for this difficulty is that when magnesia and alumina react at elevated temperatures to form magnesium aluminate there is a volume expansion. This, in the usual case, results in disruption of the refractory, often in actual expansion of the refractory, which in turn leads to low strength and high porosity.
Various prior art solutions to this problem have been proposed. U.S. Pat. No. 3,184,322, suggests the use of TiO.sub.2 as a flux, together with a finely divided alumina of specified specific gravity and malachite green decolorizing power. U.S. Pat. No. 3,333,971, suggests overcoming the problem of volume expansion upon formation of magnesium aluminate by adding zirconia to the admixture. U.S. Pat. No. 3,577,247 suggests another solution, namely initially forming magnesium aluminate with an excess of magnesia and then using this material in a brick mix containing sufficient fine alumina to react with the prereacted magnesium aluminate to ultimately form a magnesium aluminate closer to stoichiometric. It has also been suggested, in U.S. Pat. No. 3,950,504, to form magnesium aluminate spinel grain or aggregate using electrostatic precipitator dust fines as the source of alumina, such material having a relatively fine particle size and being extremely high in specific surface.
The present invention is directed to a solution of the problem of expansion upon forming magnesium aluminate spinel in situ in a refractory shape.